Research on the influence of confining pressure on coal-breaking depth with abrasive air jet

Some technical problems and narrow scope of application restrict the development of hydraulic measures to increase coal permeability，however，the abrasive air jet can mitigate the shortcomings of hydraulic measures. In or⁃ der to understand the influence of confining pressure on the coal⁃breaking depth of abrasive air jet，a new indentation fracture theory with confining pressure was proposed based on the analysis of the effect of confining pressure on inden⁃ tation force. With this new indentation fracture theory，the sequential causal relationship among abrasive and target pa⁃ rameters⁃indentation force⁃indentation size⁃transverse crack depth⁃depth of impact crater was established. Also，a model of coal breaking depth was established by considering the influence of confining pressure on indentation size and the relationship between indentation and depth of impact crater. Finally，indentation test，impact crater morpholo⁃ gy test and coal breaking depth test were carried out to verify the established model. The results show that，on the one hand，the loading confining pressure reduces the elastic deformation of coal and makes the indentation caused by the same indentation force smaller. On the other hand，the loading confining pressure makes the indentation face produce an upward force due to the existence of indentation surface，which weakens the effect of indentation force and further reduces the size of indentation. The reduction of indentation leads to the smaller size of impact crater. The effect of confining pressure on coal breaking depth is achieved by reducing the impact effect of each abrasive particle. Larger confining pressure leads to smaller indentation on coal body，less undulation on surface morphology of impact crater，shallower coal breaking depth and less quality loss. The coal breaking depth model established in this paper can accurately calculate the coal breaking depth under different confining pressure conditions， by using the depth impact crater of coal without confining pressure.